In Patent Literature 1, a structure of a holder case is shown in which a pair of cover cases and an intermediate case disposed between the pair of cover cases are provided, and two battery rows disposed in parallel are respectively tucked and held between one cover case and the intermediate case as well as between the other cover case and the intermediate case. The holder case is assembled so as to integrally connect the both cover cases and the intermediate case by fastening connection screws that penetrate the both cover cases and the intermediate case at four corners of the case.